The sphygmomanometric class of automated blood pressure monitors employ an inflatable cuff to exert controlled counter-pressure on the vasculature of the subject. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith, employs the oscillemetric methodology. In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure. Thereupon, the cuff pressure is reduced in predetermined decrements, and at each level pressure fluctuations are monitored. The resultant signals typically consist of the DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as "oscillatory complexes" or just simply "oscillations"). After suitable filtering to reject the DC component and to provide amplification, peak pulse amplitudes (PPA) above a given base-line are measured and stored. As the decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. The lowest cuff pressure at which the oscillations have a maximum value is representative of mean arterial pressure. Systolic and diastolic pressures can be derived either as predetermined fractions of mean arterial pressure, or by more sophisticated methods of direct processing of the oscillatory complexes.
The step deflation technique as set forth in the Ramsey patents has become the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale, and although development efforts have been directed to continuous deflate monitors, substantial difficulties have been encountered in securing accurate and reliable clinical results. Indeed at least one commercial blood pressure system which features the continuous deflate mode also employs a step deflation backup system, which is utilized to insure accurate results for those circumstances in which the continuous deflation proves inadequate. Thus, while efforts continue unabated for more rapid detection methods which avoid step deflations, the incremental deflate class of instrument enjoys substantial preference among clinicians.
In a contemPoraneous commonly assigned invention (see M. Ramsey, III et al. U.S. patent application Ser. No. 751,835 for "Improved Sphygmomanometric Cuff Pressurizing System" filed on even date herewith) there is disclosed and claimed apparatus for shortening the time required to inflate the pressure cuff to a level above the systelic pressure of the patent in preparation for deflation and a measurement cycle. The disclosure of such M. Ramsey. III et al. application is incorporated herein by reference.
Step deflation measurements as heretofore obtained and as exemplified by the Ramsey, III et al. patents can require at least 30 seconds to perform and occasionally as much as a full minute. The American Heart Association recommends a deflation rate for manual sphygmomanometric measurement no greater than 2-4 Torr per heart beat. For normal blood pressure measurements this results in manual determination times on the order of 30 seconds. Similarly, with conventional automated noninvasive pressure measuring devices, the time required for a normal determination is on the order of 35 seconds when the deflation steps size is the standard 5 to 6 Torr.
It is, accordingly, a primary object of the present invention to reduce the overall blood pressure measurement time by reducing the time required for cuff deflation.
It is a further object of the present invention to provide apparatus for obtaining accurate blood pressure measurement while employing significantly larger decremental steps than heretofore thought feasible over a significant portion of the deflation phase of the measurement cycle.
It is a further object of the present invention to provide apparatus capable of large decremental deflation steps within a sufficiently short interval of time to avoid skipping heart beats and thereby prolonging the measurement phase. That is, it is an object of the present invention to operate the deflation cycle at a suitable rate and tempo to coordinate with pulse rates as rapid as 100 per minute.
It is yet another object of the present invention to alleviate patient discomfort by minimizing the amount of time at which the blood pressure cuff is at higher and less comfortable pressure.